The present invention relates to a UV absorbent-containing polycarbonate resin composition and its sheet moldings. Precisely, the invention relates to such a polycarbonate resin composition having the advantages of good weather resistance and heat stability and capable of forming moldings with few colored impurities, and to sheet moldings of the composition.
As being transparent and having good heat resistance and impact resistance, polycarbonates are used in various fields of the art of resin molding, in which, however, high-temperature thermoforming has become popularized to meet the recent tendency toward large-sized and thin-walled resin moldings and toward shortened molding cycles. With that, polycarbonates are often problematic in that they are readily degraded through oxidation, discolored (yellowed) and foamed, and that the mechanical strength of their moldings is thereby lowered. Another problem with polycarbonates is that their moldings are often discolored and aged when exposed to the weather in outdoor use or to fluorescent lights in indoor use, and the mechanical strength of the aged moldings is lowered.
To solve the problems, for example, adding UV absorbents to polycarbonates has been proposed (Japanese Patent Publication Nos. 51840/1994, 45228/1994). However, the method proposed is still problematic in that the resin moldings produced therein often have gels (fish eyes) and their heat stability is still unsatisfactory, though being effective in some degree for improving the weather resistance of the resin moldings.
The object of the present invention is to provide a polycarbonate resin composition having good weather resistance and heat stability and to provide sheet moldings of the composition.
We, the present inventors have assiduously studied the problems noted above. We know that many typical UV absorbents for polycarbonates, such as benzotriazole-type UV absorbents and benzophenone-type UV absorbents are contaminated with sulfonic acid group-having compounds that are derived from their starting materials, and the concentration of the contaminants in them is high. It is therefore presumed that such sulfonic acid group-having compounds in those UV absorbents may cause oxidative degradation of polycarbonates. To confirm the fact, we made various experiments, and, as a result, have reached the present invention. Incidentally, we have found through our experiments that, when the hydroxyl content of the both terminals of the polymer chain of polycarbonates is higher than a predetermined level, then the polycarbonate moldings often have many gels (fish eyes). Based on these, we have completed the invention, which is summarized as follows:
[1] A polycarbonate resin composition comprising (A) 100 parts by weight of a polycarbonate and (B) from 0.001 to 15 parts by weight of one or more UV absorbents selected from benzotriazole-type UV absorbents, benzophenone-type UV absorbents and triazine-type UV absorbents, which is characterized in that the sulfur content of the composition is at most 50 ppb.
[2] The polycarbonate resin composition of above [1], wherein the hydroxyl content of the both terminals of the polymer chain of the polycarbonate (A) is at most 10 mol %.
[3] A sheet molding of the polycarbonate resin composition of above [1] or [2], which is produced through extrusion molding of the composition.
Benzotriazole-type UV absorbents for use in the invention are typically compounds of the following formula (I): 
wherein n is 1 or 2; and R1 indicates an alkyl group having from 1 to 10 carbon atoms.
Concretely, they include the following: 
2-(2xe2x80x2-hydroxy-5xe2x80x2-methylphenyl)benzotriazole; 
2-[2xe2x80x2-hydroxy-3xe2x80x2,5xe2x80x2-bis(xcex1,xcex1-dimethylbenzyl)phenyl]benzotriazole; 
2-(2xe2x80x2-hydroxy-3xe2x80x2,5xe2x80x2-di-t-butylphenyl)benzotriazole; 
2-(2xe2x80x2-hydroxy-3xe2x80x2-t-butyl-5xe2x80x2-methylphenyl)-5-chlorobenzotriazole; 
2-(2xe2x80x2-hydroxy-3xe2x80x2,5xe2x80x2-di-t-butylphenyl)-5-chlorobenzotriazole; 
2-(2xe2x80x2-hydroxy-3xe2x80x2,5xe2x80x2-di-t-amyl)benzotriazole; 
2-(2xe2x80x2-hydroxy-5xe2x80x2-t-octylphenyl)benzotriazole; 
2,2xe2x80x2-methylene-bis[4-(1,1,3,3-tetramethylbutyl)-6-(2N-benzotriazol-2-yl)phenol].
Benzophenone-type UV absorbents for use in the invention are typically compounds of the following formula (II): 
wherein R2 indicates an alkyl group having from 1 to 15 carbon atoms.
Concretely, they include the following: 
2,4-dihydroxybenzophenone; 
2-hydroxy-4-methoxybenzophenone; 
2-hydroxy-4-methoxybenzophenone-5-sulfonic acid; 
2-hydroxy-4-n-octoxybenzophenone; 
2-hydroxy-4-n-dodecyloxybenzophenone; 
bis(5-benzoylxe2x80x2-4-hydroxy-2-methoxyphenyl)methane; 
2,2xe2x80x2-dihydroxy-4-methoxybenzophenone; 
2,2xe2x80x2-dihydroxy-4,4xe2x80x2-dimethoxybenzophenone.
Triazine-type UV absorbents for use in the invention are typically compounds of the following formula (III): 
wherein R3 indicates a hydrogen atom, an alkyl group having from 1 to 18 carbon atoms, an alkyl group having from 2 to 6 carbon atoms and substituted with a halogen atom or an alkoxy group having from 1 to 12 carbon atoms, or a benzyl group; R4 indicates a hydrogen atom or a methyl group.
Concretely, they include 2,4-diphenyl-6-(2-hydroxy-4-methoxyphenyl)-1,3,5-triazine, 2,4-diphenyl-6-(2-hydroxy-4-ethoxyphenyl)-1,3,5-triazine, 2,4-diphenyl-6-(2-hydroxy-4-propoxyphenyl)-1,3,5-triazine, 2,4-diphenyl-6-(2-hydroxy-4-butoxyphenyl)-1,3,5-triazine, 2,4-diphenyl-6-(2-hydroxy-4-hexyloxyphenyl)-1,3,5-triazine, 2,4-diphenyl-6-(2-hydroxy-4-pentoxyphenyl)-1,3,5-triazine, 2,4-diphenyl-6-(2-hydroxy-4-octyloxyphenyl)-1,3,5-triazine, 2,4-diphenyl-6-(2-hydroxy-4-dodecyloxyphenyl)-1,3,5-triazine, 2,4-diphenyl-6-(2-hydroxy-4-benzyloxyphenyl)-1,3,5-triazine, 2,4-diphenyl-6-(2-hydroxy-4-(2-butoxyethoxy)phenyl)-1,3,5-triazine, 2,4-di-p-tolyl-6-(2-hydroxy-4-methoxyphenyl)-1,3,5-triazine, 2,4-di-p-tolyl-6-(2-hydroxy-4-propoxyphenyl)-1,3,5-triazine, 2,4-di-p-tolyl-6-(2-hydroxy-4-butoxyphenyl)-1,3,5-triazine, 2,4-di-p-tolyl-6-(2-hydroxy-4-hexyloxyphenyl)-1,3,5-triazine, 2,4-di-p-tolyl-6-(2-hydroxy-4-pentoxyphenyl)-1,3,5-triazine, 2,4-di-p-tolyl-6-(2-hydroxy-4-octyloxyphenyl)-1,3,5-triazine, 2,4-di-p-tolyl-6-(2-hydroxy-4-benzyloxyphenyl)-1,3,5-triazine, and 2,4-di-p-tolyl-6-(2-hydroxy-4-(2-hexyloxyethoxy)phenyl)-1,3,5-triazine.
Many of the above-mentioned benzotriazole-type UV absorbents, benzophenone-type UV absorbents and triazine-type UV absorbents are contaminated with sulfonic acid group-having compounds that are derived from their starting materials and side materials. For example, Japanese Patent Publication No. 19027/1964 relating to 2-(2xe2x80x2-hydroxy-5xe2x80x2-t-octylphenyl)benzotriazole discloses a method for producing the compound, and says that a sulfonated castor oil is used as the starting material for the compound. To confirm this, we analyzed a commercially-available UV absorbent that comprises the compound, 2-(2xe2x80x2-hydroxy-5xe2x80x2-t-octylphenyl)benzotriazole, and have found that it contains 60 ppm (in terms of the sulfur atom) of a sulfur compound. To clarify the oxidative degradation of polycarbonates caused by the sulfur compound, we made an experiment with a model compound, and have confirmed the reproducibility of the experiment. The details of the experiment are in the section of Examples to follow hereinunder.
Regarding benzophenone-type UV absorbents, referred to is the disclosure in Japanese Patent Laid-Open No. 62133/1983 that relates to 4,4xe2x80x2-dihydroxybenzophenone. In the method disclosed therein, the starting material used for producing the compound is a fluoroalkanesulfonic acid (concretely, trifluoromethanesulfonic acid or difluoromethanemethanesulfonic acid). Also referred to is the disclosure in Japanese Patent Laid-Open No. 235839/1990 that relates to production of 2,4-dihydroxybenzophenone. In this, used is an alkylbenzenesulfonic acid that serves as a reaction promoter.
Regarding triazine-type UV absorbents, referred to is Japanese Patent Laid-Open No. 87638/1998 in which is used a solution of aminostilbenzenesulfonic acid or disodium 4xe2x80x2-diaminostilbenzene-2,2xe2x80x2-disulfonate for producing the compounds.
The polycarbonate resin composition of the present invention comprises 100 parts by weight of a polycarbonate and from 0.001 to 15 parts by weight, preferably from 0.005 to 10 parts by weight of the above-mentioned benzotriazole-type UV absorbent and/or benzophenone-type UV absorbent, and the sulfur content of the composition is at most 50 ppb, preferably at most 30 ppb.
If the UV absorbent content of the resin composition is smaller than 0.001 parts by weight, the resin composition could not have good weather resistance. On the other hand, even if the UV absorbent content is larger than 15 parts by weight, it will be no more effective for further improving the resin composition. If the sulfur content of the resin composition is over 50 ppb, the sheet moldings of the composition will have many gels (fish eyes). Therefore, the sulfur content of the resin composition must be reduced. For reducing the sulfur content, for example, employable is purification through recrystallization.
In addition to the above-mentioned benzotriazole-type UV absorbent and benzophenone-type UV absorbent, the polycarbonate resin composition of the invention may optionally contain a hindered amine-type light stabilizer that is synergistic with the UV absorbents, and also a phenolic or phosphorus-containing antioxidant, as well as other various additives such as flame retardant, release agent, antistatic agent, lubricant, etc., not interfering with the object of the invention.
[Polycarbonates]
As having the advantages of good heat resistance, impact resistance and stiffness, aromatic polycarbonates are preferred for use in the invention. They may be not only aromatic homopolycarbonates but also various copolymers including, for example, aromatic-aliphatic copolycarbonates, aromatic polycarbonate-polysiloxane block copolymers, etc. Of such various types of polycarbonates, aromatic homopolycarbonates are more preferred for use in the invention as they are easy to produce on a mass-production scale and have good quality and well-balanced physical properties.
In general, aromatic homopolycarbonates are produced in a well-known process which comprises reacting a biphenol such as typically bisphenol A and a polycarbonate precursor such as phosgene or a carbonate compound (e.g., diphenyl carbonate). For example, in a solvent such as methylene chloride and in the presence of a known acid acceptor or a molecular weight-controlling agent and optionally a branching agent, a biphenol is reacted with a carbonate precursor such as phosgene, or transesterified with a carbonate precursor such as diphenyl carbonate to give aromatic homopolycarbonates.
The molecular chain of the thus-produced polycarbonates is terminated with a molecular weight-controlling agent such as phenol, p-t-butylphenol, octylphenol, cumylphenol, etc. In general, therefore, the polycarbonates have no hydroxyl terminals. However, in case where the reaction for polycondensation is unbalanced, the polycarbonates produced will often have hydroxyl terminals derived from the starting monomer (biphenol).
The polycarbonate to be in the polycarbonate resin composition of the invention is preferably such that the proportion of the number of terminal hydroxyl groups therein to 100 polymer chain terminals (in general, one polymer has two terminals) is at most 10 mol %, more preferably at most 8 mol % (this is referred to as terminal hydroxyl molar fraction). If its terminal hydroxyl molar fraction is larger than 10 mol %, the polycarbonate will form many gels (fish eyes) when heated. In particular, when it contains a sulfur compound derived from the above-mentioned UV absorbent, the polycarbonate forms many colored gels (fish eyes).
The molecular weight of the polycarbonate for use in the invention is preferably so controlled that the polycarbonate can be well molded into sheets. Concretely, the viscosity-average molecular weight of the polycarbonate falls between 10,000 and 50,000, preferably between 12,000 and 35,000, more preferably between 15,000 and 30,000. If its molecular weight is smaller than 10,000, the polycarbonate will have poor impact resistance; but if larger than 50,000, the polycarbonate will be difficult to mold.
[Sheet Moldings]
The polycarbonate resin composition of the invention can be molded into sheets in a mode of extrusion molding, and the sheets have good weather resistance and heat resistance. The sheets can be formed into various types of moldings in a mode of vacuum forming, etc. As being resistant to heat, the sheet moldings are used for pouches or containers for retortable foods, and therefore must not have colored gels (fish eyes). Accordingly, the sheet moldings of the polycarbonate resin composition of the invention are favorable for pouches or containers for retortable foods.